This research project deals with the study of regional distribution of pulmonary ventilation in the non-homogeneous lung. It is intended to test the hypothesis that respiratory frequency and tidal volume should affect the regional distribution of alveolar ventilation in different ways depending on the state of parenchyma and airways. The aim of this research is to study in living dogs, the effects of respiratory frequency and tidal volume, mean lung volume and interegional mixing, on the regional distribution alveolar ventilation under three distinct conditions of non-homogeneous lung mechanisms: 1) Large airway obstruction created with a bronchoscopically inserted, known crossectional area orifice; 2) unilateral bronchoconstriction produced by histamine in aerosol, and 3) unilateral low compliance caused by lung lavage. Alveolar ventilation will be assessed by measuring the local turnover rate, from the lung airspaces, of radio-active Nitrogen- 13 during washout maneuvers using a 2D positron camera, or a stationary positron emission ring tomographic camera. This unique ring camera creates 3 dimensional reconstructions of the lungs with excellent spacial resolution. Regional distribution of gas transport created with several settings of conventional and high frequency small-tidal volume mechanical ventilation will be measured. Simultaneously, regional mechanical characteristics and oscillatory flow rates will be measured or estimated, using a combination of newly developed intra-airway pneumotachographs, novel techniques with the positron cameras and computer model of lung mechanics. Theoretical models that incorporate the mechanical and gas transport characteristics of the lung at a regional level will be developed and tested against the data obtained experimentally. It is expected that the results of these experiments will improve our understanding of the basic physiologic mechanisms of gas transport within the lung and ultimately contribute in developing a rationale for selecting the respiratory parameters in the mechanically ventilated patient. This project will also provide the background knowledge needed to develop improved diagnostic tools of regional pulmonary function.